Bacterial methanotrophs drive the formation of a seasonal anoxic benthic nepheloid layer in an alpine lake

• Published in: Limnology and oceanography Vol. 59; no. 4; pp. 1410 - 1420
• Main Authors: Blees, Jan, Niemann, Helge, Wenk, Christine B., Zopfi, Jakob, Schubert, Carsten J., Jenzer, Joël S., Veronesi, Mauro, Lehmann, Moritz F.
• Format: Journal Article
• Language: English
• Published: Waco, TX John Wiley and Sons, Inc 01-07-2014; American Society of Limnology and Oceanography
• Subjects: Animal and plant ecology; Animal, plant and microbial ecology; Biological and medical sciences; Fresh water ecosystems; Fundamental and applied biological sciences. Psychology; General aspects; Synecology; Freshwater environment; Lakes; Bacteria; Anoxia; Seasonal variation; Benthic zone
• ISSN: 0024-3590; 1939-5590
• DOI: 10.4319/lo.2014.59.4.1410

We investigated the formation and microbial composition of a seasonal benthic nepheloid layer (BNL) in the eutrophic, monomictic southern basin of Lake Lugano. During stratification, a BNL developed at the sediment– water interface and progressively expanded 20–30 m into the water column, following the rising oxic–anoxic interface. The dominance of the fatty acids C16:1ω5, C16:1ω6, C16:1ω7, and C16:1ω8, with δ13C values between −62‰ (ω6) and −80‰ (ω7), suggests that the BNL was composed primarily of Type I aerobic methane oxidizing bacteria (MOB). Indeed, MOB contributed > 75% to the fatty acid carbon pool in the fully developed BNL, with cell densities up to 8.5 × 10⁵ cells mL−1. In ex situ incubation experiments, CH₄ turnover rate coefficients were up to 2.1 d−1, which translates into potential CH₄ oxidation rates as high as 20 mmol m−3 d−1 under in situ CH₄ concentrations. CH₄ oxidation was limited by the diffusive supply of O₂, and O₂ consumption by aerobic CH₄ oxidation (up to 13.1 mmol m−2 d−1) appears to be the primary driver of the seasonal growth of the BNL and expansion of the hypolimnetic anoxic zone. Methanotrophic activity at the interface between oxic and anoxic water masses can actuate the formation of a BNL, which in turn functions as an effective microbial CH₄ filter in the water column, preventing CH₄ transport to surface waters and evasion to the atmosphere. In situ biomass production by methanotrophic bacteria may represent, in addition to sediment resuspension and detritus trapping, a novel BNL formation mechanism.